Free space optical communication tracking with electronic boresight compensation and co-boresighted transmit and receive optics

Abstract

Exemplary embodiments described herein include a bi-directional Free Space Optical (FSO) communication unit that may be used in a multi-node FSO communication system. The bi-directional FSO unit may include a co-boresighted optical unit such that received and transmitted beams are coincident through a common aperture. Embodiments described herein may be used to correct or accommodate the alignment errors of the received and transmitted beams.

The invention claimed is: 
     
       1. A Free Space Optical (FSO) communication unit, comprising:
 co-boresighted beam steering unit configured to transmit and receive optical beams along a coincident path external to the FSO communication unit; 
 a receiver beam detector configured to determine a position of the receive optical beam on the receiver beam detector; 
 a transmit beam source for transmitting the transmit optical beam towards a direction; 
 a Pointing, Acquisition, and Tracking (PAT) control system,
 wherein the PAT control system controls a direction of the transmit optical beam, and directs the transmit optical beam based on the position of the receive optical beam based on the position of the receive optical beam and an offset command, 
 the offset command value is calculated for the FSO communication unit based on an error between the position of the receive optical beam and the direction of the transmit optical beam, the error caused by an optical misalignment of at least one optical component of the FSO communication unit. 
 
 
     
     
       2. The FSO communication unit of  claim 1 , further comprising one or more optical components to create the receive optical beam such that a diameter of the receive optical beam on the receiver beam detector is 40-60% of a diameter of the receiver beam detector. 
     
     
       3. The FSO communication unit of  claim 2 , wherein the one or more optical components includes a diffuser. 
     
     
       4. The FSO communication unit of  claim 1 , wherein the co-boresighted beam steering unit comprises co-boresighted FSO Tx/Rx optics such that the transmit and receive optical beams are directed along an approximately coincident path within the FSO communication unit. 
     
     
       5. The FSO communication unit of  claim 1 , wherein the PAT control system communicates with the co-boresighted beam steering unit to direct the co-boresighted beam steering unit such that the position of the receive optical beam is located at a non-centered value that results in the transmit optical beam being pointed coincident with the receive optical beam external to the FSO communication unit. 
     
     
       6. The FSO communication unit of  claim 1 , wherein the receiver beam detector is a quad-cell photo detector. 
     
     
       7. The FSO communication unit of  claim 6 , further comprising a second receiver beam detector that has a detection surface with a response independent of location of Rx beam incidence. 
     
     
       8. The FSO communication unit of  claim 7  further comprising:
 a separator to direct the transmit optical beam and the receive optical beam along separate paths. 
 
     
     
       9. The FSO communication unit of  claim 8  wherein the separator is a dichromic beam splitter to separate the coincident path of the transmit optical beam and the receive optical beam. 
     
     
       10. The FSO communication unit of  claim 8 , further comprising a beam splitter such that the receive optical beam is split between the receiver beam detector and the second receiver beam detector. 
     
     
       11. The FSO communication unit of  claim 1 , wherein the PAT control system directs the transmit optical beam based on the position of the receive optical beam by driving the transmit optical beam to be directed parallel to the receive optical beam based on the position of the receive optical beam and the offset command. 
     
     
       12. The FSO communication unit of  claim 1 , wherein the at least one optical component of the communication unit causing the error is located optically between the transmit beam source and the receiver beam detector. 
     
     
       13. The FSO communication unit of  claim 1 , wherein the offset command value is further calculated based on a point-ahead bias, an angular bias due to the delay in the speed of light, to account for time delay between the FSO communication unit and a remote FSO communication unit. 
     
     
       14. An FSO communication system, comprising the FSO communication unit of  claim 1  and a remote FSO communication unit separated over a distance and directed toward each other to communicate data over a line of sight, further comprising a communication link between the FSO communication units such that the error may be calculated for the FSO communication unit from an offset detected on the receiver beam detector of the FSO communication unit when the transmit optical beam of the FSO communication unit is centered on a beam detector of the remote FSO communication unit. 
     
     
       15. A method of sending data from a co-boresighted beam steering unit of a Free Space Optical (FSO) communication unit on a transmitted optical beam at a first wavelength and receiving data at the co-boresighted beam steering unit on a received optical beam at a second wavelength different from the first wavelength, comprising:
 aligning paths of the transmitted optical beam with the received optical beam such that the paths extend through the co-boresighted beam steering unit and are coincident exterior to the FSO unit; 
 calculating an error between a position of the received optical beam and a direction of the transmitted optical beam, the error caused by an optical misalignment of at least one optical component of the FSO communication unit; and 
 positioning the transmitted optical beam according to an alignment of the received optical beam and the error. 
 
     
     
       16. The method of  claim 15 , further comprising separating the transmitted optical beam and the received optical beam to traverse separate paths within a portion of the FSO unit. 
     
     
       17. The method of  claim 15 , wherein aligning comprises measuring a position of an optical component with an alignment sensor to determine the position of the received optical beam relative to its center. 
     
     
       18. The method of  claim 17 , further comprising adding a value proportional to the error to a signal from the alignment sensor to create an adjusted signal that offsets the error, and positioning the transmitted optical beam by centering the adjusted signal. 
     
     
       19. The method of  claim 15 , wherein the at least one optical component of the communication unit causing the error is located optically between a transmit beam source of the FSO communication unit and a receiver beam detector of the FSO communication unit. 
     
     
       20. The method of  claim 15 , wherein the transmitted optical beam is further positioned according to a point-ahead bias, an angular bias due to the delay in the speed of light, to account for time delay between the FSO communication unit and a remote FSO communication unit. 
     
     
       21. The FSO communication unit of  claim 1 , wherein the PAT control system further controls a direction of the receive optical beam, and directs the receive optical beam towards the stationary receiver beam detector based on the offset command value. 
     
     
       22. The FSO communication unit of  claim 1 , wherein the calculated offset command value is a constant value calculated prior to operational use of the FSO communication unit. 
     
     
       23. The method of  claim 17 , wherein the alignment sensor remains at a fixed position relative to the at least one optical component of the FSO communication unit. 
     
     
       24. The method of  claim 15 , wherein the method of  claim 15  is a calibration method done prior to operational use, and further comprising:
 receiving a second received optical beam, during operational use; 
 sending a second transmitted optical beam, during operational use; and 
 positioning the second transmitted optical beam according to an alignment of the second received optical beam and the error, during operational use.